(core) API fix for a bug that treated 0 as null

Summary: Bug with 'records' endpoint that was treating 0 as null.

Test Plan: Modified tests

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3262

app/server/lib/DocApi.ts

@@ -272,7 +272,7 @@ export class DocWorkerApi {
       const fieldNames = new Set<string>(_.flatMap(records, r => Object.keys(r.fields ?? {})));
       const result: BulkColValues = {};
       for (const fieldName of fieldNames) {
-        result[fieldName] = records.map(record => record.fields?.[fieldName] || null);
+        result[fieldName] = records.map(record => record.fields?.[fieldName] ?? null);
       }
       return result;
     }